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Background  Syncope is a sudden transient loss of consciousness and postural tone caused by cerebral hypoperfusion. The most common form is vasovagal syncope ( VVS ). Presyncopal progressive early hypotension in older  VVS  patients is caused by reduced cardiac output ( CO ); younger patients have reduced systemic vascular resistance ( SVR ). Using a priori criteria for reduced  CO  (↓ CO ) and  SVR  (↓ SVR ), we studied 48 recurrent young fainters comparing subgroups of  VVS  with  VVS ‐↓ CO ,  VVS ‐↓ SVR , and both  VVS ‐↓ CO &↓ SVR .    Methods and Results  Subjects were studied supine and during 70‐degrere upright tilt with a Finometer to continuously measure blood pressure,  CO , and  SVR  and impedance plethysmography to estimate thoracic, splanchnic, pelvic, and calf blood volumes, blood flows, and vascular resistances and electrocardiogram to measure heart rate and rhythm. Central blood volume was decreased in all  VVS  compared to control.  VVS ‐↓ CO  was associated with decreased splanchnic blood flow and increased splanchnic blood pooling compared to control. Seventy‐five percent of  VVS  patients had reduced  SVR , including 23% who also had reduced  CO . Many  VVS ‐↓ SVR  increased  CO  during tilt, with no difference in splanchnic pooling, caused by significant increases in splanchnic blood flow and reduced splanchnic resistance.  VVS ‐↓ CO &↓ SVR  patients had splanchnic pooling comparable to  VVS ‐↓ CO  patients, but  SVR  comparable to  VVS ‐↓ SVR . Splanchnic vasodilation was reduced, compared to  VVS ‐↓ SVR , and venomotor properties were similar to control. Combined splanchnic pooling and reduced  SVR  produced the earliest faints among the  VVS  groups.    Conclusions  Both ↓ CO  and ↓ SVR  occur in young  VVS  patients. ↓ SVR  is predominant in  VVS  and is caused by impaired splanchnic vasoconstriction.

Mechanisms of Vasovagal Syncope in the Young: Reduced Systemic Vascular Resistance Versus Reduced Cardiac Output